The present invention relates to a defect notification method and apparatus in a multipoint ATM network. More particularly the present invention relates to a defect notification method and apparatus in a multipoint ATM network for preventing defect notification signals from being generated in an overlapped manner from a plurality of leaf nodes in the network and preventing undesired defect notification signals from merging on a root node in the network.
Recommendation I. 610 "Broadband (B-)ISDN Operation and Maintenance Principles and Functions" approved by WTSC (World Telecommunications Standard Conference) March 1993 (hereinafter abbreviated as ITU-TI. 610) by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) stipulates a defect notification method in a "point-to-point ATM connection" but do not stipulate a defect notification method in a "multipoint ATM connection". According to the defect notification method in a point-to-point ATM connection (hereinafter explained with reference to VP: virtual path), a device that has detected a section defect (or a defect of the lower layer as viewed from the ATM layer) sends a VP - AIS (VP-Alarm Indication Signal) cell in the downstream direction of the ATM connection that is multiplexed on the defective section. The VP - AIS cell is terminated and detected by a first device that is an end point of the ATM connection, and it is recognized that the ATM connection is defective. In response to the reception of the VP - AIS cell, the first device sends a VP - FERF (VP - far end receive failure) cell to the ATM connection which forms a pair with respect to the defective ATM connection and is in the opposite direction relative thereto. The VP - FERF cell is terminated and detected by a second device that is an end point of the ATM connection in the opposite direction. This makes it possible to recognize the presence of a defective point in the ATM connection which is on the source side as viewed from the above device.
Without anything stipulated under ITU - TI. 610, however, the defect notification method in the above-mentioned point-to-point ATM connection must be directly expanded to the multipoint ATM connection. FIGS. 8A-B illustrate the flow of notification signals occurring when a defect notification method in the conventional point-to-point ATM connection is adapted to a multipoint connection shown in FIG. 6. The multipoint ATM connection shown in FIG. 6 comprises a source end point (hereinafter referred to as root), a plurality of sink end points (hereinafter referred to as leaves), one or more connecting points with branch, and connecting points without branch, and does not lose general performance of the multipoint ATM connection (VPC: virtual path connection or VCC: virtual channel connection).
In the example of FIG. 6, the end point of the multipoint connection has a root R and leaves L1, L2, L3 and L4. As the connecting points, there are points B1, B2 and B3 with branch and points C1 and C2 without branch. Here, the root R is, for example, an exchange included in the network, the leaves L1 to L4 are terminals or exchanges, and the connecting points B1 to C2 are exchanges or transmission equipment. The connections correspond to VPC or VCC, and are divided into several links by the above-mentioned end points and connecting points. In this example, the links include S1 (between R and C1), S2 (between C1 and B1), S3 (between B1 and C2), S4 (between C2 and B2), S5 (between B2 and L1), S6 (between B2 and L2), S7 (between B1 and B3), S8 (between B3 and L3), and S9 (between B3 and L4). The connections can be defined to be a connection in the downward direction from the root to the leaves and a connection in the upward direction from the leaves to the root. The connection in the downward direction of FIG. 6 is shown in FIG. 7A and the connection in the upward direction is shown in FIG. 7B. In this specification, the connections of links Si (i=1 to 9) in the downward direction are expressed as Sdi (i=1 to 9) and in the upward direction are expressed as Sui (i=1 to 9).
According to the conventional defect notification method as shown in FIG. 8A, when the lower layer becomes defective, for example, in a link Sd1, the connecting point C1 adjacent to the defective point on the downstream side inserts a AIS cell in the downstream connection. Then, as shown in FIG. 8B, the leaves L1 to L4 that have received the AIS cell sends the VP - FERF cell back to the upstream connection. However, the disadvantage of the conventional defeat notification method is that a plurality of VP - FERF cells sent from the leaves L1 to L4 merge at the connecting points where the AIS cell is branched, thereby multiplying the band by the number of merging points at the connection points (B1, B2, B3) in the connection in the opposite direction and multiplying the band at the root R which is the end point by the number of leaves.